This invention is a synthetic speech generating apparatus in which a power supply to a speech generating section, which comprises a speech synthesizer and an amplifier or the like, is cut off except during speech generation. For example, this invention relates to a noise reduction circuit for apparatuses such as speech timepieces and speech calculators or the like.
More specifically, this invention relates to a noise reduction circuit in which a logic circuit of the speech synthesizer is stopped by controlling and stopping a fundamental clock oscillator before the logic circuit of the speech synthesizer malfunctions due to a drop in the power supply voltage when the power supply to the synthetic speech generator is cut off. In other words, the invention is a noise reduction circuit which prevents noise generation immediately after completion of speech generation.
Conventionally, in synthetic speech generating apparatuses of the above type, a power controller was inserted between the speech generator and the battery power supply. By driving and stopping this controller by means of control signals from the central processing unit, savings in battery power could be obtained since power would be supplied to the speech generator only during speech generation.
However, in conventional apparatuses, there is a tendency for the power supply voltage to the speech generator to drop gradually due to the effect of the protective electrolytic capacitor connected between a power controller and the speech generator. This protective capacitor is used for the purpose of preventing speech generation from being interrupted when connections to the battery power supply are cut off for some reason while power is supplied to the synthetic speech generator.
For this reason, a problem existed in that the logic circuit of the synthetic speech generator malfunctioned when the voltage dropped below a certain value. Because output from this circuit is digital, the ON-OFF condition of the flip-flops deviate by the number of data bits provided in a subsequent stage of the circuit, causing completely irrelevant data to be supplied to the power amplifier from the flip-flop array. Furthermore, if the power amplifier is in operation during the drop in voltage, this data is amplified and emitted as noise from the speaker.